I have a 64 with the 292 I have hand made headers with 3" collector. 3" pipe running into flowmasters with 3" out. My pipes run all the way out the back.
I'm not in it for H.P. but I can do 80 mph with no problem.
My rear end is 3.00 so highway cruisin is just fine. The sound is a deep throaty
sound. Here's the best picture I have of it. I guess it all depends on what your trying to achieve.

 

I could get all complicated figuring out how much CFM the induction system flows in order to see how much the exhaust system flows at various rpm levels. Suffice it to say my modified 302-4V that resides in my Cougar has a 2.25" exhaust system based on all that. Car doesn't see past 5000 rpm and the numbers told me that 2.5" had excess capacity that the engine wouldn't fill. No way a stock 292 should be larger than 2.25" and if really stock a 2.00" dual exhaust would probably be fine. This is another area where many think bigger just like CFM for their carb and duration for their cam.

Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:
<table style="text-align: center;" border="1" cellpadding="5" cellspacing="0"> <tbody> <tr> <td height="38" width="116" align="CENTER">Pipe Diameter (inches)</td> <td width="73" align="CENTER">Pipe Area (in2)</td> <td width="78" align="CENTER">Total CFM (est.)</td> <td width="95" align="CENTER">Max HP Per Pipe</td> <td width="132" align="CENTER">Max HP For A Dual Pipe System</td> </tr> <tr> <td height="17" align="CENTER">1 1/2</td> <td align="CENTER">1.48</td> <td align="CENTER">171</td> <td align="CENTER">78</td> <td align="CENTER">155</td> </tr> <tr> <td height="17" align="CENTER">1 5/8</td> <td align="CENTER">1.77</td> <td align="CENTER">203</td> <td align="CENTER">92</td> <td align="CENTER">185</td> </tr> <tr> <td height="17" align="CENTER">1 3/4</td> <td align="CENTER">2.07</td> <td align="CENTER">239</td> <td align="CENTER">108</td> <td align="CENTER">217</td> </tr> <tr> <td height="17" align="CENTER">2</td> <td align="CENTER">2.76</td> <td align="CENTER">318</td> <td align="CENTER">144</td> <td align="CENTER">289</td> </tr> <tr> <td height="17" align="CENTER">2 1/4</td> <td align="CENTER">3.55</td> <td align="CENTER">408</td> <td align="CENTER">185</td> <td align="CENTER">371</td> </tr> <tr> <td height="17" align="CENTER">2 1/2</td> <td align="CENTER">4.43</td> <td align="CENTER">509</td> <td align="CENTER">232</td> <td align="CENTER">463</td> </tr> <tr> <td height="17" align="CENTER">2 3/4</td> <td align="CENTER">5.41</td> <td align="CENTER">622</td> <td align="CENTER">283</td> <td align="CENTER">566</td> </tr> <tr> <td height="17" align="CENTER">3</td> <td align="CENTER">6.49</td> <td align="CENTER">747</td> <td align="CENTER">339</td> <td align="CENTER">679</td> </tr> <tr> <td height="17" align="CENTER">3 1/4</td> <td align="CENTER">7.67</td> <td align="CENTER">882</td> <td align="CENTER">401</td> <td align="CENTER">802</td> </tr> <tr> <td height="17" align="CENTER">3 1/2</td> <td align="CENTER">8.95</td> <td align="CENTER">1029</td> <td align="CENTER">468</td> <td align="CENTER">935</td> </tr> </tbody> </table> NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.
The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.


How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos

 

Sometimes I'll read about people suggesting an H pipe when running dual exhaust. How is that better? Have been thinking about those rams horns that Mummert sells. Y blocks sound great when the exhaust is setup nice.

 

I run these on my y block with long tube headers, I love them and they almost make it to the back of my cab
http://www.summitracing.com/parts/bi...2flt/overview/

 

H pipe, X Pipe or crossover pipe. The reasoning is that it equalizes the pressure between the banks thus making the engine more efficient. The air pressure going through the carb and manifold is equal to each bank going into the engine and should be equalized on the exit to be efficient and obtain best gas mileage. Most factory dual exhaust systems have an H pipe from the get go. Equal in, equal out. It usually gets in the way when dropping a tranny but increases mileage, how much I don't know but the experts that designed the system must see it as significant.

If you are blasting down the drag strip with your headers open you are not concerned with ultimate gas mileage, just how quick you can make that quarter mile and how much octane you need to keep the timing advanced enough without blowing it up!

I guess you could say that's the difference for performance tuning and tuning for economy. along with other factors also.

Check this discussion about pipes here: http://www.jalopyjournal.com/forum/t...exhaust.86649/

 

Thanks for the link! I pretty much want to do exactly what the OP is doing, though maybe with rams horns. There is some fitment issues with the oil dipstick though not a major deal.

 

In this link is info about numerous dyno tests performed on the same engine, on the same dyno. It's a Y block, but all sorts of header and manifold designs were tested. Exhaust pipe diameter, length, mufflers/no mufflers, as well as an H-pipe were tested. Testing is done on an engine that is not stock, but it is not particularly wild, either. It is meant to represent a fairly typical modified engine that the average hot rodder can build.

Exhaust test to end all tests?

I think you will be fine with 2 1/2" pipes. I don't think you can find an X pipe smaller than 2 1/2", either.